Creep rupture life

Figure 9.6. (a) The temperature dependence of the flow stress for a Ni-Cr-AI superalloy containing different volume fractions of y (after Beardmore et al. 1969). (b) Influence of lattice parameter mismatch, in kX (eflectively equivalent to A) on creep rupture life (after Mirkin and Kancheev... 

Above the threshold, deformation occurs as a consequence of direct particle interaction. Several mechanisms of interaction have been suggested solution-precipitation flow of fluid between particles and cavity formation at the particle matrix interface. These theories of creep suggest several rules to improve creep behavior (1) increase the viscosity of the matrix phase in multiphase materials (2) decrease the volume fraction of the intergranular phase (3) increase the grain size (4) use fiber or whisker reinforcement when possible. As the creep rupture life is inversely proportional to creep rate, lifetime can be improved by improving creep resistance. 

Fig. 6.114 Monkman-Grant plot shows that NT 154 and NT164 follow the same linear relationship, indicating that the same mechanism is controlling creep rupture life for both materials [46]. With kind permission of John Wiley and Sons...

Voorhees analysis assumes that the creep-rupture life of a vessel under complex stressing is controlled by an equivalent stress, J, termed the shear-stress invariant. This average stress is also known as the octahedral shear stress, the effective stress, the intensity of stress, and the quadratic invariant. The theory for the biaxial-stress condition was developed by Von Mises (205), and this theory was further developed to apply to the triaxial-stress condition independently by Hencky (206, 207, 208) and by Huber (209). A derivation of the relationship between the equivalent stress, /, and the three principal stresses, /i, /2, and/s where /i > /2 > /s was given by Eichinger (210). The relationship between these stresses is ... 

Creep strain/time curves have been determined for a range of SiCf/SiC type composites, " for various types of SiC fibre and for other CFCMCs. However, the faetors eontrolling the creep properties of different composites have usually been diseussed by reference to only a few standard parameters, such as the minimum creep rate (Sm), and the creep rupture life (tf). For this reason, the present projeet seeks to demonstrate that evidence derived from analyses of creep curve shape is relevant to interpretation of the detailed manner in which specific material variables influence strain aecumulation and damage evolution during tensile creep of CFCMCs. 

Figure 8.2 Yield strength (a) and creep-rupture life (b) of several US austenitic steels [54] 316, fine-grained and Nb stabilized 347HFG, Ti-stabilized D9/PCA, and multistabilized + ultrafine...

Creep properties of Haynes 230 (stars) at 850°C in air and in vacuum compared to the creep behaviour of (a) Alloy 617 (o/rupture time curves after Ennis [19]) and (b) Hastelloy X (o/creep rupture life curves after Hada et al. [20]). 

Decarburisation Carbon removal causes the dissolution of carbides degrading the creep properties of carbide-hardened alloys. Occurring at high temperature, this damage is fast and deep. Shindo and Kondo [14] show that the creep rupture life of Hastelloy XR shortens greatly under decarburising helium. 

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